Abrasive grit materials are well known in the art and are commonly used as blast media in sand blasting operations. Typical applications for abrasive grit blast media include cleaning building exteriors, or removing surface coatings such as surface oxides on castings. In applications such as these, minor surface damage caused by the blasting operation is not of concern and abrasive grits which are relatively aggressive in removing surface coatings may be used. However, other applications exist in which the surface damage which results from the use of traditional abrasive grit blast media is unacceptable.
One example of a particularly sensitive application involves the military and commercial aircraft industry. The removal of paint from exterior aircraft surfaces is periodically required in order to allow technicians to inspect the underlying metal for corrosion and also for cosmetic purposes, such as a change in corporate identity. However, aircraft surfaces clad with aluminum or made of composite materials are particularly sensitive to damage caused by blast media.
Aircraft surfaces are typically clad with pure aluminum, which is softer than the underlying aluminum alloy the cladding is designed to protect. The soft layer of aluminum cladding is particularly susceptible to damage and erosion by traditional blast paint stripping operations, which can leave a very rough surface finish that is unacceptable in most situations. Such damage to the aluminum cladding can necessitate the replacement of whole sections of an aircraft's skin. Furthermore, methods used for stripping aluminum clad structures may not be acceptable for structures made of composite materials such as fiberglass, graphite and aramid (Kevlar) systems which are even more sensitive to damage from blast media.
Prior art blast media used in aircraft applications include synthetic abrasive grit materials composed of thermosetting resins such as epoxy resin, urea resin, unsaturated polyester resin, alkyd resin, or harder resins such as polystyrene, polycarbonate or acrylic. Although less aggressive than traditional blast media, plastic blast media are sufficiently aggressive when used in paint stripping applications that skilled handling by trained personnel is required in order to avoid permanent damage to aircraft skins. Moreover, use of such blast media can still result in reduced fatigue life, increased crack propagation and high surface roughness to the extent that such media is unacceptable for repeated use on aluminum clad or composite structures.
Gentler methods of paint removal involve the use of chemical solvents such as methylene chloride or phenol. However, stringent environmental legislation and health standards have rendered the use of such volatile organic chemicals in paint stripping operations both inconvenient and prohibitively expensive. In some cases, companies have postponed aircraft exterior surface maintenance programs rather than attempt to deal with the regulations and costs associated with chemical stripping.
A series of related patents, owned by the assignee of the present invention, describe an entirely new type of abrasive grit material. U.S. Pat. Nos. 5,066,335, 5,360,903 and 5,367,068 describe glass-like polysaccharide abrasive grit particles made from conventional starches, such as wheat and corn starches. The abrasive grit particles are comprised of polysaccharide molecules annealed into a glass-like solid in which moisture is occluded within the interstices of the matrix of polysaccharide molecules. The polysaccharide molecules may be partially cross-linked to increase the complexity of the matrix of polysaccharide molecules within the glass-like polysaccharide grit particles.
The glass-like polysaccharide abrasive grit compositions described by U.S. Pat. No. 5,367,068 enjoy several advantages over the prior art abrasive grits. Polysaccharide abrasive grits are less aggressive than plastic blast media, thereby avoiding the damage to aluminum clad structures caused by traditional blast stripping methods. As a result, use of polysaccharide abrasive grits is more forgiving and requires less skill in handling than the use of plastic blast media. Use of polysaccharide abrasive grits allows aluminum clad aircraft skins to be subjected to repeated paint-and-strip operations in comparison to plastic blast media. By way of contrast, some aircraft manufacturers limit paint stripping operations using plastic blast media to only once in the lifetime of an aircraft, due to the potential damage to the aircraft's surface. Furthermore, polysaccharide abrasive grits are acceptable for use with both aluminum and composite materials, allowing for complete nose-to-tail paint stripping operations of aircraft. Finally, abrasive grits made of polysaccharides such as wheat starch are non-toxic and biodegradable, reducing the health risks and disposal problems often associated with paint stripping operations. These advantages over other methods have allowed the glass-like polysaccharide starch grits described in U.S. Pat. No. 5,367,068 to gain wide acceptance in the industry for removing coatings from aircraft.
One shortcoming to the use of the prior art polysaccharide grit particles is that they are hygroscopic. When placed in contact with liquid water, the prior art grit particles tend to cake and agglomerate, absorbing water to the extent that they lose their integrity and form a non-particulated mass. Once in this state, the grit particles cannot be recovered by drying and are no longer usable as blast media.
The hygroscopic nature of prior art blast media can create operational problems when these materials come into contact with any water present in the blast equipment. In general, blasting operations involve the use of an air compressor to create an air stream in which the abrasive particles are suspended. The flow of abrasive particles is then concentrated and directed through a nozzle onto the surface to be treated. Water can enter into the blast equipment where incompletely dried compressed air is used, or where rapid depressurization causes condensation within pressure pot system and air supply lines. This problem is particularly acute in humid environments.
The failure to keep the blast equipment free of water can result in caking and agglomeration of the blast media, rendering it useless for its intended purpose, and can render the blast equipment ineffective by obstructing valves and clogging apertures within the blasting apparatus. Thus, use of the prior art polysaccharide blast media requires special precautions to ensure a dry air source for the compressor and blast equipment designed to minimize condensation in pressurized zones of the equipment. Moreover, these concerns are not limited to the equipment problems encountered during use. Caking and agglomeration of blast media may also arise during long term storage of the prior art polysaccharide blast media in extremely humid environments.
Another problem particular to the aircraft industry involves infiltration of abrasive grit blast media into the interior of the aircraft during blast stripping operations. The failure to properly mask and seal all joints and seams on the exterior surface of an aircraft prior to blast stripping may permit some abrasive grit material to infiltrate spaces containing sensitive systems, which could cause potential damage to such systems. Similarly, small cracks on the exterior surface may become filled with blast media, inter alia providing an inconsistent surface for receiving a coating. These problems are compounded by the difficulty in detecting the abrasive grit particles due to their small size and generally transparent or translucent appearance. The problems of detection can be exacerbated by the inaccessibility and poor lighting conditions of some interior spaces within aircraft. As a result, clean up of abrasive grit material from the interior of an aircraft can be laborious and time consuming, sometimes requiring substantial disassembly of the affected portion of the aircraft.
Thus, it would be desirable to provide polysaccharide grit particles which maintain their integrity when placed in contact with liquid water and which retain their usability as blast media after they are dried. In addition, there is a need for an abrasive blast media which may facilitate detection and clean up of abrasive grit particles that infiltrate the joints and seams present in treated surfaces.